This application relates generally to a vehicle tire inflation system for continuously supplying air from the vehicle""s axles to the rotating tires. In one of its aspects, it relates to a system of this type having an improved rotary union connection to transmit pressurized air from an axle end to the surrounding rotating hub cap by means of a tube received at one end near the center of the axle end and at the second end near the center of the hub.
The commercial trucking industry is a large market in need of automatic tire inflation systems. Tractor and trailer rigs can typically have 18 tires, and because of their constant use and cost it can be justifiable for a fleet to purchase tire inflation systems for tire pressure maintenance. Properly inflated tires can greatly extend tire life and reduce hazardous blowouts. To meet the needs of the trucking industry, a low cost, low maintenance, and durable rotary union is essential.
A typical rotary union for such a system has one member fixed in relation to the end of the axle and another fixed in relation to the hub cap. Some will incorporate a smooth graphite face seal to serve as the dynamic wear seal and others use elastomer seals. Most existing designs have relatively large dynamic sealing surfaces and their size is proportional to the surface speed of the wearing surface. Large seals can compensate for misalignment between the axle and hub cap since the large cross-section will expand and contract with every half rotation. A larger seal helps guarantee sealing but generates more heat, costs more, and generally wears more quickly. Some tire inflation systems add expensive controls to provide air only momentarily to the rotary unions to extend their life, limiting the effectiveness of the inflation system.
As shown in U.S. Pat. No. 5,769,979, the rotary union is able to use a small dynamic seal by compensating for bearing wobble and misalignments between the hub and axle through the use of a pivoting rigid tube extending between two elastomer seals. Distortion of the elastomer seals while the tube pivots can impair their sealing ability and longevity. Providing sufficient clearance around the seals so the rigid tube can pivot freely can also be a detriment to a seal""s performance since proper backing isn""t provided for the seal. A rigid tube also is vulnerable to permanent bending during handling and installation.
The object of this invention is to provide a system having an improved rotary union for durability and wear, wherein, as compared with other systems, the tube is flexible so as to compensate for bearing wobble and misalignment between the axis of the axle and the axis of the hub. Elongate openings in the rotary union""s members affixed to the hub cap and to the axle, guide the flexible tube through the seal rings contained within the members. Thus, this invention also allows the ends of the tube to remain axially aligned with the seal rings to provide more reliable seals, and thus allow for greater misalignment than a pivoting rigid tube, as in U.S. Pat. No. 5,769,979. The improvement also allows for greater durability during installation and operation since the tube will not be weakened or destroyed if it is bent.
The flexible tube allows for another novel embodiment of the invention. Since substantial lengths of the tube ends are contained in the elongate openings, redundant seal rings can be added along the tube""s length contained in the elongate openings. This provides the ability for air to be vented to the atmosphere in the event of a leak in the outermost seal contained within the member affixed to the hub cap. A redundant seal and a vent to the atmosphere between the two seals in this will prevent air from entering the lubrication compartment between the axle end and hub cap.
In the preferred and illustrated embodiments of the invention, the tire inflation system includes a rotary union having a first member which is fixed in relation to a hollow axle, a second member fixed in relation to a hub, both having an elongate opening therethrough, and a flexible tube which is received at its first end within a first seal ring carried about the opening in the first member and received at its second end within a second seal ring carried about the second member near the hub""s axis, at least one of which forms a dynamic seal during the hub""s rotation, wherein misalignment between the hub axis and the axle axis is compensated for primarily by the flexibility of the tube. The seals prevent air passage from around the outer diameters of the flexible tube, and the elongate opening in each member aligns the end of the flexible tube so that segment of the flexible tube which passes through the seal ring remains coaxial with the opening during rotation.
In order for the system to inflate a tire, the vehicle requires a frame mounted source of air pressure to supply air as needed to the tires. Pressurized air passes through a system of valves and through the hollow axle to pressurize the seal ring of the rotary union""s first member. Air then passes through the flexible tube to pressurize the seal ring carried about the opening in the rotary union""s second member, and finally is sent from the second member through a pressure conduit to the tire.
The flexible tube is held in alignment with the small seals in each rotary union member, at least one of which is a dynamic, and a third dynamic seal can be added in line with the seal in the second member to divert air through a vent between the seals directly to the atmosphere in the event the first dynamic seal leaks. Thus, this improvement prevents unwanted pressurization of the lubricated bearing compartment, which when pressurized can destroy the wheel seal and purge the lubricant causing bearing failure.